The present invention is directed toward an antenna system for use in an electronic security system and, more particularly, toward such an antenna system which includes individually fed multiple loops.
Electronic security anti-pilferage systems are widely known for the detection of the unauthorized removal of items tagged by a detectable target containing a resonant circuit, saturable magnetic wire strip or mechanically resonant magnetic material. The basic concepts for such theft detection systems are described in U.S. Pat. Nos. 3,810,147; 3,973,263; 4,016,553; 4,215,342 and 4,795,995 and many others.
A variety of antenna configurations have been designed to be used with anti-pilferage systems. Practical transmitter antenna designs typically have one or more loops of wire carrying alternating current to generate an electromagnetic field. The receiver antenna is also typically one or more loops of wire which receives small distortions or disturbances in the electromagnetic field caused by the detectable target as it passes through the interrogation zone between the transmitter and receiver antennas. A desirable feature of the receiver antenna system is for it to be sensitive to signals originating within the interrogation zone or at distances which are small relative to the antenna dimensions and be insensitive to or cancel noise and spurious signals which originate at distances far from the interrogation zone, i.e. at distances that are large compared to the antenna dimensions.
Similarly, it is desirable for the transmitter antenna to create a strong local field in the interrogation zone and minimize or cancel fields created at large distances from the interrogation zone. Such transmitter antenna far field cancellation is beneficial in meeting RF emission levels as may be required by the FCC or other similar regulatory agencies.
Far field cancellation is demonstrated by Heltemes in U.S. Pat. No. 4,135,183 with an hourglass or figure eight design receiver and transmitter antenna. Lichtblau in U.S. Pat. No. 4,243,980 proposes twisting a single conductor to form a multiloop far field cancelling design. In U.S. Pat. No. 4,251,808, a conductive shield is added enclosing the twisted loops to provide electrostatic shielding. And in U.S. Pat. No. 4,751,516, Lichtblau proposes driving symmetrical half sections of twisted loops.
All of the far field cancelling multiple loop antennas in the above-cited patents inherently suffer from an inability to achieve good amplitude balance and exact phase opposition at high frequencies. Twisted loops inherently shift current phase relative to the driving source as one moves away from the source causing unbalance in the loops furthest from the source. Shielded loops exaggerate the problem. Additionally, the above-mentioned inherent phase unbalance can, in some frequency-swept detection systems, cause undesirable effects which manifest as distortions to the signals normally associated with the field disturbance targets or markers.